What this test measures
Thyroid Stimulating Hormone (TSH) is produced by the anterior pituitary in response to circulating thyroid hormone levels — falling when T3 / T4 are high and rising when they are low. This negative feedback makes TSH the single most sensitive marker of primary thyroid dysfunction.
Third-generation ("ultrasensitive") TSH assays measure down to 0.01 mIU/L. This sensitivity is essential for distinguishing mildly suppressed from completely suppressed TSH — critical in detecting and titrating treatment of hyperthyroidism, levothyroxine over-replacement and subclinical disease.
Why it matters
TSH is the first-line screen for thyroid dysfunction and is the recommended single test by the American Thyroid Association, the Endocrine Society and the Indian Thyroid Society. A normal TSH essentially rules out primary thyroid disease in a non-pregnant adult.
Thyroid disorders affect roughly 1 in 10 Indian adults, and symptoms (fatigue, weight changes, mood, hair, menstrual irregularities) are non-specific and often dismissed. Universal screening from age 35 is recommended by Indian Thyroid Society guidelines. In pregnancy, trimester-specific TSH ranges apply — TSH > 2.5 in the first trimester is increasingly treated.
How to prepare
No fasting required. Morning sample preferred for consistency since TSH has a mild diurnal rhythm (slightly higher early morning). If on levothyroxine, take it after the blood draw. Stop biotin / cosmetic supplements 48–72 hours before. Continue other medications.
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| TSH (3rd Generation Ultra-Sensitive) (mIU/L)[1][2] | 0.4 – 4.0 (general adult) · Trimester 1 of pregnancy: 0.1 – 2.5 · Trimester 2/3: 0.2 – 3.0 | Suppressed TSH (< 0.4) suggests overt or subclinical hyperthyroidism, over-replacement on levothyroxine, recent acute illness, or biotin interference. Fully suppressed (< 0.01) almost always indicates clinically meaningful hyperthyroidism. Confirm with FT4 and FT3. | Raised TSH (> 4.0) suggests primary hypothyroidism. Mild raises (4–10) are usually subclinical and re-checked in 6–8 weeks. Higher values (> 10) almost always need levothyroxine. Marked elevations also occur on recovery from acute illness, in some thyroiditis stages, and with non-adherent thyroxine replacement. |
Ultrasensitive TSH — decision bands
| TSH (mIU/L) | Interpretation | Action |
|---|---|---|
| < 0.01 | Fully suppressed | Confirm hyperthyroidism — FT4 + FT3 |
| 0.01 – 0.4 | Suppressed | Subclinical or early overt hyperthyroidism — confirm + monitor |
| 0.4 – 4.0 | Normal | Euthyroid |
| 4.0 – 10 | Mildly raised | Subclinical hypothyroidism — re-check 6–8 weeks; treat if pregnant / symptomatic |
| > 10 | Markedly raised | Overt hypothyroidism — start levothyroxine |
| Trimester 1 > 2.5 | Pregnancy-specific raised | Treat as per pregnancy thyroid guidelines |
Frequently asked questions
Is "ultrasensitive" TSH different from a regular TSH?
It is the same test, run on a third-generation assay that measures down to 0.01 mIU/L. All modern Indian labs use this version by default. Older second-generation assays could not distinguish mildly suppressed from fully suppressed TSH.
Do I need to fast?
No. A morning sample is preferred for consistency since TSH has a mild diurnal rhythm, but fasting is not required.
My TSH is 5 — do I need treatment?
A mildly raised TSH (4–10) with normal Free T4 is "subclinical hypothyroidism". Most cases are re-checked in 6–8 weeks. Treatment is usually started if you are pregnant, planning pregnancy, anti-TPO positive, or symptomatic with persistent results.
Should I take my levothyroxine on the test day?
Take it after the blood is drawn. Taking it 1–2 hours before transiently affects readings.
Will biotin affect TSH?
Yes — high-dose biotin (in cosmetic supplements at 5–10 mg) classically causes falsely low TSH and falsely high T4 / T3. Stop 48–72 hours before testing.
How often should TSH be repeated?
Every 5 years for low-risk adults from age 35 (Indian Thyroid Society recommendation), annually for women planning pregnancy, every 6–8 weeks during levothyroxine titration, and every 6–12 months once stable.
Why does pregnancy need a different TSH range?
Pregnancy hCG cross-reacts with the TSH receptor and lowers TSH. Trimester-specific ranges (1st trimester: 0.1–2.5; 2nd/3rd: 0.2–3.0) prevent over-diagnosis of hyperthyroidism and under-diagnosis of hypothyroidism that affects fetal brain development.
Related Hormones / Endocrine tests
Tests commonly ordered alongside TSH - ULTRASENSITIVE, or that help interpret an unexpected result.
Sources & references
- American Thyroid Association — Thyroid Function Tests · accessed 2026-05-30T00:00:00.000Z
- Endocrine Society Clinical Practice Guidelines · accessed 2026-05-30T00:00:00.000Z
- NIH MedlinePlus — TSH Test · accessed 2026-05-30T00:00:00.000Z
- Indian Thyroid Society — Consensus · accessed 2026-05-30T00:00:00.000Z
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